Let me give you something else to chew on:
He literally slaughtered his way through the World Eaters' and Emperor's Children in a single night. Orginally, both Legions had been fighting on a planet but the planet was becoming so cold that it threatened to freeze them as night fell. So...they called a ceasefire and sought shelter. Kharn was so pissed at what he considered cowardice that he went a got a flamethrower before proceeding to set the city they were in on fire. He burned his way through city, incinerating or hacking apart anyone who tried to stop him and did this all night.
Kharn wrecked such havoc that he
shattered both Legions. What was left of them split into warbands, never to cohere again as a single unified force. At least until the First War of Armegeddon, where the World Eaters came back together to fight beside Daemon Prince Angron, their Primarch. Kharn's action earned him the title '
Betrayer' for killing his own Legion kin.
If it hasn't sunken in, Kharn was already the one of the elite of humanity's greatest fighting force. A fighting force that nearly conquered the Galaxy. He was already one of the most dangerous Space Marines alive, and pretty much in the same league as Sigismund (
greatest non-Primarch duelist who never lost until fighting Abaddo), Abaddon (
a Marine so skilled and powerful that it was considered a feat to delay him and his forces for a FEW MINUTES during the Heresy), and Sevatar (
a guy who can fight through Lion El'Jonso's flipping flagship (which is the size of a city!) single handedly, quickly killing all in his way and hold Sigismund in a formal duel for 30 HOURS and only lost cuz he broke a rule). He has come even further than that. Hell, before the end of the Heresy, Kharn apparantly scored a
millio kills before collapsing on a pile of corpses in a single battle. He is a machine that does nothing but fight and kill. He decimates opponents with hundreds to thousands of years of experience, who dedicate themselves to combat and war, like it's nothing.
Superman is skilled but he's meeting a guy whose track record in combat dwarfs his by several magnitutdes, who wields a weapon that can end him in one good hit and who lives for nothing but fighting.
To borrow a phrase from the Space Wolves: someone will have to inform Lois Lane that her husbands now sleeps on the red snow.